Voltage controlled oscillators (VCOs) are commonly employed in phase-locked loops (PLLs) that often provide clock recovery functionality in serial link communications. As its name implies, a VCO provides an oscillating signal with a frequency that depends on a control voltage. The frequency range of the VCO, i.e., the range of possible output frequencies for a particular range of input voltages, is an important aspect of a VCO. The ability to achieve a greater frequency range of a VCO allows greater opportunity to support more frequency ranges in a single serial link. While increasing the gain increases the frequency range, a problem exists with a corresponding increase in the noise/jitter component as the gain increases. Delay cells of a VCO typically do not cover a large frequency range while maintaining low VCO gain. Conventional methods of covering double the frequency range include the use of switchable dividers on output phases and the use of current starved cells that allow variation in tail current. Unfortunately, these methods also have their drawbacks, including increased complexity of the switchable dividers approach and increased random jitter problems with the tail current approach.
Accordingly, a need exists for a VCO delay cell with an adjustable frequency range that maintains substantially constant gain and avoids the problems of random jitter increase and increased complexity. The present invention addresses such a need.